1. Field of the Invention
The present invention relates to a code division multiple access (CDMA) mobile wireless communication system, and more particularly to a power control for the CDMA communication system.
2. Description of the Related Art
In a mobile wireless communication system, a handoff operation must be performed when a base station moves from one region to another. During a soft handoff operation, a selector in a base station controller (BSC) generally performs an outer loop power control of the base stations belonging in an active set. FIG. 1 shows a portion of a typical mobile wireless communication system.
Referring to FIG. 1, each base station 20xcx9c40 receive a signal from a mobile station (MS) and performs a cyclic redundancy check (CRC) on the received frame. The base stations 20xcx9c40 report the result of the CRC to a selector 12 of a BSC 10. Utilizing the present CRC results and previous target frame error rates determined based upon a history of CRC results, the selector 12 adjusts power control threshold values of the base stations 20xcx9c43 so that each BS can independently maintain a target frame error rate.
A power control threshold value is adjusted in steps by a range of ratio K. Whether the power control threshold values are adjusted up or down is determined by a predetermined target frame error rate F. For example, in order for the selector 12 to maintain a target frame error rate of 1%, the ratio K of the down step for lowering the power control threshold value is determined to be K=(1/F)xe2x88x921, i.e. K=(1/0.01)xe2x88x921=99. Thereafter, a corresponding inner loop power control threshold value is determined.
In the outer loop power control method as described above, the ratio of the up/down step size to adjust the power control threshold value is determined in the same manner regardless of the state in which a mobile station may be in. In other words, the value of the ratio is determined identically during a soft handoff and when a soft handoff is not being performed.
If a frame error ratio of the base station in the soft handoff active set increases during a soft handoff, the selector 12 executes a selection combining operation, to meet the target frame error rate. However, if the ratio of the up/down step is adjusted during a soft handoff in the same manner as when a soft handoff is not being performed, the power control threshold may abruptly increase. This causes a mobile station to transmit a signal using more power than necessary, resulting in the decrease of the whole capacity for users of the communication system.
As a result, if the ratio of the up/down step size is identically adjusted during a soft handoff and when a soft handoff is not being performed, the base stations cannot perform an accurate power control with respect to the mobile station.
Accordingly, an object of the present invention is to solve at least the problems and disadvantages of the related art.
An object of the present invention is to provide a more effective cellular communication system.
Another object of the present invention is to provide a more effective outer loop power control method for a cellular communication system.
A further object of the present invention is to provide an outer loop power control method which can variably adjust the ratio of an up/down step size for a power control threshold value of base stations.
Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.
To achieve the objects and in accordance with the purposes of the invention, as embodied and broadly described herein, the outer loop power control method includes performing an outer loop power control in a normal mode according to a fixed target frame error rate and determining a corresponding inner loop power control threshold value; and performing a soft handoff mode by determining a target frame error rate according to the number of base stations which belong to a soft handoff active set and by determining a corresponding ratio of up/down step size to an outer loop power control threshold value.
In the outer loop power control method, an outer loop power control is performed independently during a soft handoff, with respect to the base stations in the active set. Also, in the outer loop power control method, a plurality of base stations which belong to a soft handoff active set check errors of received frames depending on whether when a soft handoff from a mobile station is initiated. The base station which belong to the soft handoff active set then determines a target frame error rate according to a result of the checked frame error, checked previous frame error, and the number of base stations which belong to the active set. Finally, the base stations each determine a ratio of an up/down step size for the power control threshold value according to the determined target frame error rate.